A filter system for cleaning flue gas is known from U.S. Pat. No. 5,318,755 A wherein a barrier filter element of a honey-comb structure is accommodated in a housing together with a separate catalyst element. The raw gas is first passed through the barrier filter element into a clean gas space and then passed from the clean gas space through the catalyst element which likewise is of a honey-comb structure. The catalyst element is substantially coextensive with the filter element. Clean gas is subsequently discharged from the housing in a direction substantially parallel to the flow direction within the filter element and the catalytic element.
Catalytic filter modules in the form of cylindrical filter candles and a filtration system comprising same within a filter vessel has been proposed in the U.S. Pat. No. 6,863,868 B1 for hot gas filtration. The filter candles have a unitary porous body of sintered particles, said particles being coated with a catalytic layer on their surfaces. The upstream or feed surface of the filter candles is covered by a porous membrane retaining fines. Clean gas accumulates within the filter candles and is withdrawn therefrom into a common clean gas space from where it is discharged from the vessel of the system.
The prior art catalytic filter modules in the form of cylindrical unitary filter candles are typically made of ceramic materials. Conventional catalytic ceramic filter candles have a length of typically 2 or sometimes even 3 m which, because of the mechanical properties of the sintered ceramic material and the mechanical stress during operation of the filter systems cannot be further increased. Therefore, the filter area of the individual candles is limited, and for treating higher volume flow rates thousands or ten thousands of filter candles are necessary. Very big filter vessels or several vessels to be operated in parallel are needed in order to accommodate such a large number of catalytic filter candles.
However, the volume flow rates in hot gas filtration applications may range up to about 1 million m3/h or more which are too high for the use of such conventional catalytic ceramic filter candles.